Hard disk drives are common information storage devices essentially consisting of a series of rotatable disks that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers, are typically carried by and embedded in a slider body that is held in a close relative position over discrete data tracks formed on a disk to permit a read or write operation to be carried out. In order to properly position the transducer with respect to the disk surface, an air bearing surface (ABS) formed on the slider body experiences a fluid air flow that provides sufficient lift force to “fly” the slider and transducer above the disk data tracks. The high speed rotation of a magnetic disk generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body which enables the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface through this self-actuating air bearing.
Some of the major objectives in ABS designs are to fly the slider and its accompanying transducer as close as possible to the surface of the rotating disk, and to uniformly maintain that constant close distance regardless of variable flying conditions. The height or separation gap between the air bearing slider and the spinning magnetic disk is commonly defined as the flying height. In general, the mounted transducer or read/write element flies only approximately a few micro-inches above the surface of the rotating disk. The flying height of the slider is viewed as one of the most critical parameters affecting the magnetic disk reading and recording capabilities of a mounted read/write element. A relatively small flying height allows the transducer to achieve greater resolution between different data bit locations on the disk surface, thus improving data density and storage capacity. With the increasing popularity of lightweight and compact notebook type computers that utilize relatively small yet powerful disk drives, the need for a progressively lower flying height has continually grown.
As shown in FIG. 1 an ABS design known for a common catamaran slider 5 may be formed with a pair of parallel rails 2 and 4 that extend along the outer edges of the slider surface facing the disk. Other ABS configurations including three or more additional rails, with various surface areas and geometries, have also been developed. The two rails 2 and 4 typically run along at least a portion of the slider body length from the leading edge 6 to the trailing edge 8. The leading edge 6 is defined as the edge of the slider that the rotating disk passes before running the length of the slider 5 towards a trailing edge 8. As shown, the leading edge 6 may be tapered despite the large undesirable tolerance typically associated with this machining process. The transducer or magnetic element 7 is typically mounted at some location along the trailing edge 8 of the slider as shown in FIG. 1. The rails 2 and 4 form an air bearing surface on which the slider flies, and provide the necessary lift upon contact with the air flow created by the spinning disk. As the disk rotates, the generated wind or air flow runs along underneath, and in between, the catamaran slider rails 2 and 4. As the air flow passes beneath the rails 2 and 4, the air pressure between the rails and the disk increases thereby providing positive pressurization and lift. Catamaran sliders generally create a sufficient amount of lift, or positive load force, to cause the slider to fly at appropriate heights above the rotating disk. In the absence of the rails 2 and 4, the large surface area of the slider body 5 would produce an excessively large air bearing surface area. In general, as the air bearing surface area increases, the amount of lift created is also increased. Without rails, the slider would therefore fly too far from the rotating disk thereby foregoing all of the described benefits of having a low flying height.
As illustrated in FIG. 2, a head gimbal assembly 40 often provides the slider with multiple degrees of freedom such as vertical spacing, or pitch angle and roll angle which describe the flying height of the slider. As shown in FIG. 2, a suspension 74 holds the HGA 40 over the moving disk 76 (having edge 70) and moving in the direction indicated by arrow 80. In operation of the disk drive shown in FIG. 2, an actuator 72 moves the HGA over various diameters of the disk 76 (e.g., inner diameter (ID), middle diameter (MD) and outer diameter (OD)) over arc 75.
As the flying height of the slider decreases, interference between the slider ABS and the disk surface increases in frequency. This interference is often called “head-disk interference” (HDI). It includes both direct contact between the slider and the disk, and indirect contact through debris, lubricant, etc. on the disk surface. The greater the HDI, the more wear and tear on the slider and its ABS. HDI can damage the read-write head directly, or cause catastrophic failure by disabling the air bearing.
To combat the problems associated with HDI, a tolerance is set for the flying height of the slider. Thus, it is assumed that if the measured flying height of the slider is too low, then there will be too much HDI, adversely affecting the operation of the hard-disk drive. As stated above, however, the lower the flying height of the slider, the greater the data capacity for the drive.
One problem seen with using a flying height tolerance to control HDI is that as the flying height of conventional sliders is reduced, the tolerances become tighter. A typical flying height for a slider is a few nanometers. Variations in surface topography for the disk and slider, vibration in both surfaces, and debris/lubricant accumulating, migrating, and dropping off both surfaces add complexity to the measurement of flying height at any particular time.
As sliders have become smaller and smaller, it becomes more difficult to include traditional spacing transducers such as capacitance probes, photonic probes, etc. Furthermore, testing the flying height of a slider over a transparent disk as is known in the art causes additional problems. Since the transparent disk and the magnetic disk used in the drive differ in mounting conditions, disk roughness, and electrostatic attraction caused by “tribo-charging,” the measure of flying height over the transparent disk may not correlate to the flying height over the magnetic disk. Also, the measurement resolution of the flying height at such a low flying height can be very poor, and measurement of flying height over the transparent disk can cause contamination of the slider or electrostatic discharge (ESD) damage.
Since flying height varies over particular areas of the slider, it has been suggested to measure flying height over a very small region of the air bearing surface. For example, “magnetic spacing” would be the space directly under the read-write head and may be measured by analyzing the read-back signal from the read-write head. During measurement of the magnetic spacing, the disk speed, air pressure, gas composition in the slider-disk interface is controlled to reduce the flying height of the slider. Flying height may also be reduced by applying a DC voltage across the slider-disk interface. The change in magnetic spacing can be calculated e.g., using the Wallace equation. Thus, a slider that can have its magnetic spacing reduced by a significant amount is presumed to have an adequate flying-height margin. To implement this method of measuring magnetic spacing requires relatively expensive equipment and does not guarantee that other parts of the slider have impacted the disk.
Rather than inferring the flying height of the slider, some methods known in the art attempt to detect the HDI directly. For example, friction between the slider and disk can be measured by either a strain-gauge or by motor power consumption. Slider-disk impact can be measured from the acoustic emission of the slider or by a piezo-electric sensor. Perturbation of the slider position relative to the disk can be detected through amplitude, frequency, or phase modulation in the read-back signal. The equipment needed to measure these parameters can be expensive and may not be able to detect mild head-disk interference. One other method for HDI detection is to detect temperature changes in the read-write head. As with the magnetic spacing measurements described above, the only area being monitored is the read-write head, and other areas on the slider may impact the disk.
In view of the above, there is a need to measure HDI directly while avoiding the cost and measurement problems seen in the art.